Method of blackening stainless steels



United States Patent 3,142,592 METHOD OF BLACKENING STAINLESS STEELS Anthony J. Certa, Norristown, and George L. Schnable, Lansdale, Pa., assignors, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Delaware No Drawing. Filed Oct. 31, 1961, Ser. No. 148,816

Claims. or. 148-614) This invention relates generally to a method of chemically oxidizing iron and its alloys and more particularly to a method o fi blackening stainless steel.

The term stainless steel as used in the specification and claims is defined as an iron-based alloy containing 12-30% chromium. The stainless steels to which my invention is applicable are those containing in excess of about 65% iron and having 12-30% chromium with or without nickel and with or without other alloying ingredients, such as manganese, silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium, titanium, and the like as Well as those impurities ordinarily found in stainless or chromium steels.

The blackening of stainless steels has always proved a difficult task and has heretofore been accomplished only by costly electrolytic techniques or by high temperature chemical oxidation. Moreover, many of the prior art processes are productive of a coating which is only loosely adherent to the metal substrate with the consequence that the coating cracks, chips, or spalls off under moderate stress applications. Furthermore many of these coatings are porous, lack hardness and durability and are insufiiciently resistant to corrosion. The so-called hydroxide coatings, for example, are so soft as to be of no practical value.

It is accordingly an object of this invention to provide a simple, inexpensive process for blackening stainless steel which overcomes the deficiencies of the prior art.

A further object of the invention is to provide a low temperature chemical oxidation procedure for blackening stainless steel which is productive of a tough, strongly adherent corrosion resistant coating.

It is a still further object of this invention to provide a stainless steel blackening process peculiarly adapted to the production of a surface coating capable of uniform and consistent reproduction.

The foregoing and other objects and advantages of the invention will become apparent as the specification proceeds.

As a result of the strength and corrosion resistance characteristics of stainless steels they have found wide use in both commercial and military applications. It becomes desirable, and often necessary, in numerous of these applications to provide the part with a black surface coating, either for architectural or decorative purposes, to reduce reflection, as for example when the part is used in connection with optical devices, to increase thermal radiation, or to impart or enhance the parts corrosion resistant properties.

Briefly described, the invention in its preferred aspect relates to a process for blackening stainless steels which contain a minimum of about 65% iron. The process consists simply of immersing the stainless steel part in an alkaline solution saturated with mercuric oxide (HgO). Although the process is operative when employing less than saturation concentrations of mercuric oxide this is not the preferred practice. The coatings produced by this technique are dense and uniform in thickness and composition and are strongly adherent to the base metal. The coating is highly resistant to corrosion and can withstand several minutes immersion in concentrated nitric acid. The coating can, however, be dissolved in concentrated hydrochloric acid. It has been found, through extensive experimentation, that iron and alloys of iron having an iron content above the critical value of about 65% are blackened by this process. In establishing this value a number of alloys were tested including stainless steel 5 types 302, 303, 304, 416, 446. Also tested were Nilvar, Nichrome, Niron No. 52, Carpenter 20, Sealmet No. 4 and Kovar. All of the listed AISI stainless steels were blackened. The balance of the listed alloys however, were not affected. It was also found that the process did not blacken pure nickel or pure chromium. The composition of these alloys is set out. in Table 1.

Table I Nominal composition, percent Alloy 0, Mn, Si, Cr Ni Other(s) max max max.

AISI type 302 .15 2. 00 1.00 16-18 6-8 AIsI type 303 .15 2.00 1.00 17-19 8-10 .15 min.S.

AISI type 304 .08 2.00 1.00 18-20 8-12 AISI type 416 .15 1.25 1.00 12-14 .15 min. Se. AISI type 440 .20 1. 50 1.00 23 27 .25 max. N.

36 Bal. Fe. 60 Bal. Fe. 51 Bal. Fe. 29 Bal. Fe. 42 Bal. Fe. 28 18% oo, Bal.

The exact'mechanism of the reaction however is not known. Blackening will not occur in caustic alone, mercuric oxide, or some other mercury compound which is decomposable in alkali to mercuric oxide, such for example as Hg(NO is essential to the reaction. Although mercuric oxide is essentially insoluble in water andin alkaline solutions, the equilibrium constant for the reaction of mercuric oxide with the hydroxyl radical indicates that a small concentration of mercurate ions does exist. The equilibrium constant given for this reaction is K=3.l5 10- It was first thought that blackening occurred as a result of the formation of the very insoluble black mercuric sulfide (solubility product equal to 3 10 at C.) as a by-product of the interaction of mercurate ions with sulphur present in the stainlms steels. Numerous tests, however, have disproved this initial theory. Samples of blackened stainless steels were baked at 630 C. in nitrogen for 30 minutes, or in hydrogen at 600 C. for thirty minutes with no visible change in the appearance of the coating. Black mercuric sulphide undergoes an exothermic transition to a bright red form at 386 C. Further to test this initial assumption blackened stainless steel samples were immersed in a concentrated sodium sulfide (Na S) solution at room temperature for 24 hours or in a boiling solution for 10 minutes with no visible reaction. Mercuric sulfide is reported to be soluble in sodium sulfide solution. Furthermore mercury was not detected in a spectrographic analysis of material scraped from a stainless steel rod blackened by this process. The sensitivity limit of this test for mercury was about 0.2%.

It is therefore thought to have been conclusively proved that the black coating produced through practice of the method teachings of this invention is not mercuric sulfide.

Solubility tests in various acids and the temperature at which the black compound was removed in a hydrogen atmosphere indicate that the coating is iron-oxide (Fe O It is believed that although mercuric oxide (HgO) is essentially insoluble in water and in alkaline solutions, that the mercuric oxide combines with the monovalent hydroxyl radical to form the mercurate ion HHgO which acts as a catalyst permitting formation of the black Fe O form of iron oxide.

While the precise theory underlying this invention is 3 not fully understood, the above explanation appears fully compatible with that which has been established experi mentally.

Regardless of the accuracy of the theory, however, it has been found that pure iron and alloys of iron having an iron content in excess of 60% inclusive of that class of alloys known as stainless steels can be blackened by immersion in an alkaline solution containing more than trace amounts of mercuric oxide.

As indicated above, it has further been experimentally determined that the process is only applicable to stainless steels having an iron content in excess of about 65% and that the reaction only occurs in an alkaline solution in the presence of mercuric oxide.

One exemplary formulation of reactants used in the blackening of stainless steels such, for example, as types 302, 303, 304, 416 and 446 was a 5 normal potassium hydroxide solution containing 1 gram of mercuric oxide per 100 ml. Under these conditions most of the mercuric oxide remains undissolved producing a solution saturated with mercuric oxide. To facilitate temperature controls the solution was brought to its boiling point of about 106 C. Under these conditions the parts took on a dark black appearance after a ten minute period of immersion. The quantity of HgO required to saturate a 5 normal boiling solution of potassium hydroxide is about .5 gram per liter.

Using the formulation described above a preferred procedure for blackening stainless steel, such for example as a part made of type 416 martensitic stainless steel having the composition shown in Table l, is to first cleanse or pickle the part so as to remove any oxide film or residue of dirt or grease remaining from previous handling or manufacturing processes. One preparation effective in removing surface contaminants of the type mentioned is an aqueous solution of 20% nitric acid and 1% hydrochloric acid. After the stainless steel part has been thoroughly cleaned it is withdrawn from the pickling bath and rinsed. The surface condition of the article prior to treatment is largely determinative of the final appearance of the surface coating. The part (which may take the form, for example, of a metallic eyelet or ferrule such as used in the fabrication of semi-conductor stems of the type described in US. Patent 2,988,853 filed August 16, 1957 and assigned to the assignee of the present invention) is then immersed in the treatment bath. The solution as previously indicated is conveniently maintained at its boiling point of 106 C. The reaction takes from 10 to 30 minutes depending on the thickness of black coating desired. As previously mentioned parts oxidized in this manner exhibit a jet black surface coating which is uniform, dense, strongly adherent to the metal substrate and highly corrosion resistant. Additionally, the oxide is readily Wettable by vitric materials permitting the formation of an hermetic glass-to-metal seal. Seals are desirably fabricated in accordance with the method disclosed and claimed in the copending application of Henry P. Beerman and Ford K. Clarke, bearing Serial No. 654,907, filed April 24, 1957, and assigned to the assignee of the present invention. This method, briefly described, consists of compacting fusible insulating material, such as glass in granulated form within the eyelet to such a degree as to result in a mechanical integration of the particles. This serves to produce sufficiently intimate contact between the glass and oxidized surface portions of the eyelet to effect, on subsequent heating to a temperature in the region of approximately 900 C., hermetic juncture of the glass with the oxide interlayer.

One practice of the present invention is to use a 5 N potassium hydroxide solution; other concentrations and alkalies are also effective. For example it has been found that the process works equally well using sodium, calcium or lithium hydroxide in solutions of various normality. Furthermore, any mercury salt may be used which Will decompose in alkali to form HgO. Mercuric nitrate, for example, when placed in an alkaline solution decomposes forming mercuric oxide which in turn combines with the hydroxyl ion present in the solution to form the more complex mercurate ion. The mercuric nitrate provides an excellent vehicle for achieving optimum dispersion of mercuric oxide in the solution. It has also been found that satisfactory blackening can be achieved by use of any quantitatively measurable amount of mercury reactant ranging from just more than a trace amount to saturation level.

It has been observed that the speed of blackening in addition to being temperature dependent is also affected by the quantity of mercuric oxide in solution. Optimum results are obtained by maximizing both the temperature and concentration of the mercury reactant. Accordingly it is desirable that the oxidizing bath be brought to the boiling point in order to insure that the reaction proceeds as rapidly as possible. This also provides a convenient method of achieving temperature control. By this technique uniform, consistent results for a given immersion time can be easily maintained. At the boiling point (106 C.) of a 5 normal solution of KOH saturated with HgO the surface of the part being blackened, such as the eyelet mentioned previously, can be oxidized to a dense uniform black in only fifteen minutes. Temperature dependence of the reaction is dramtically illustrated by the fact that an immersion time of four days is required to effect blackening of stainless steel if this formulation is maintained at a depressed temperature of about 30 C. It will accordingly be seen that as the temperature and/or concentration of HgO increases the reaction time decreases proportionately.

To summarize, the invention relates to a process for blackening iron and alloys of iron containing about 65% of iron in the alloy, the process having particular application to stainless steels. The method consists of chemically oxidizing surface portions of a body by treating those portions of the body on which the coating is desired with an alkaline solution containing mercuric oxide. To optimize results it is desirable to use a sufficient quantity of the mercury-bearing compound to achieve a satura tion concentration of mercuric oxide. By utilization of this unique but simple formulation stainless steels which have heretofore been capable of being blackened only by electrolytic techniques or high temperature oxidation procedures, may be blackened by simple immersion, either by bath or spray techniques, in a relatively low temperature chemical solution, formulated from inexpensive ingredients.

While preferred practice, illustrative of the method and concepts of the present invention, have been depicted and described it will be understood by those skilled in the art that the invention is susceptible to changes and modifications without departing from the essential concepts of, and that such changes and modifications are contemplated as come within the scope of the appended claims.

We claim:

1. The method of oxidizing surface portions of a metallic body composed of a stainless steel alloy containing iron in excess of about 65 of its total composition, which comprises chemically oxidizing said surface portions through immersion in a composition consisting essentially of an alkaline solution containing mercuric oxide in an amount ranging from trace to saturation levels.

2. The method of oxidizing stainless steel parts containing iron in excess of about 65 of the alloys composition, which comprises: treating surface portions of such parts with a. composition consisting essentially of an alkaline solution containing mercuric oxide in an amount ranging from trace to saturation levels; maintaining said composition during treatment at a reactive temperature; and continuing said treatment for a period of time suflicient to produce an oxide coating of desired thickness.

3. The method of oxidizing surface portions of a body of stainless steel containing iron in excess of about 65% of the alloys composition, which comprises: immersing said surface portions in a composition consisting essentially of an alkaline solution saturated with mercuric oxide; maintaining said composition during treatment at a reactive temperature; and continuing treatment for a period of time sufficient to produce on said surface portions an oxide coating of desired thickness.

4. In the oxidation of iron and of stainless steels containing iron in excess of about 65 of the alloys total composition, the process which comprises: immersing the metal in a composition consisting essentially of an alkaline solution saturated with mercuric oxide; maintaining said composition during treatment at the boiling point of said solution; and continuing treatment for a period of time sufficient to produce an oxide coating of desired thickness.

5. In the oxidation of iron and of stainless steels containing iron in excess of about 65 of the alloys total composition, the process which comprises chemically oxidizing the metal in a composition consisting essentially of an alkaline solution containing mercuric oxide in an amount ranging from trace to saturation levels.

References Cited in the file of this patent Seidell: Solubilities of Inorganic and Organic Compounds, vol. 2, supplemented to 2nd edition, D. Van Nostrand, 1928, p. 1297. (Copy in Group 110.) 

1. THE METHOD OF OXIDIZING SURFACE PORTIONS OF A METALLIC BODY COMPOSED OF A STAINLESS STEEL ALLOY CONTAINING IRON IN EXCESS OF ABOUT 65% OF ITS TOTAL COMPOSITIONS, WHICH COMPRISES CHEMICALLY OXIDIZING SAID SURFACE PORTIONS THROUGH IMMERSION IN A COMPOSITION CONSISTING ESSENTIALLY OF AN ALKALINE SOLUTION CONTAINING MERCURIC OXIDE IN AN AMOUNT RANGING FROM TRACE TO SATURATION LEVELS. 